The military of the Frankfurt was first formally organised in 1727. As part of the Holy Roman Empire the city was part of the collective defence arrangements along with the other member states, having a fixed quota of troops to provide to the local district (Kreis) contingent. Frankfurt was one of the larger states in the Upper Rhine (Oberrhein) kreise. States could choose to provide troops or to pay cash in kind, Frankfurt chose the former and maintained a small standing army in order that that this contingent would always be ready for service. The Upper Rhine Kreis had a contribution of 3 infantry regiments, Frankfurt's troops were part of the 3rd Regiment along with those of the Princes of Nassau, the Counts of Solms, Leiningen and others.

The size of the armed forces was theoritically around 860 men divided into:

· 3 stadt companies (the 3rd was added in 1749)

· 7 kreisecontingents companies (the 7th was added in 1746)

· 1 Feuerwerker Company

· 1 Veterans company

The stadt companies were the regular guard forces within the city itself; while the kreisecontingents companies were those that would be sent to Rechsarmee during wartime, there was 1 Grenadier and 6 musketeer companies. Strength was around 90 men per company during wartime The Fuerwerkers were the artillery detachment, by 1792 they numbered 61 men and were noted for their extremely poor training and equipment.

Alongside the regular armed forces there was also a militia organised - each of the 14 city quartiers had a militia company (bürghercompagnie or quartiercompagnie). Strength was 1 Captain, 1 Lieutenant, 1 standard-bearer, 20-25 NCO's, 2 bodyguards (leibs-schützen), 2 drummers and all the residents and citizens from each quartier forming the ranks. From these 14 companies a number of picked constables formed an artillerie company; these men were excused normal fatigue duties but were responsible for the ringing of the alarms during fires and other law and order duties. The bürghercompagnien would be called during times of civil disorder or fire, during ceremonial occassions such as elections or coronations; during wartime they would take over garrison duties from the regular units. The militia was noted as being very badly organised and equipped. There was also 3 companies Geleitsreiter (Escort riders) of 30 men each - first founded in 1657 to guard the election after the death of Ferdinand III they were an entirely ceremonial bodyguard-style unit largely composed of young men from wealthy families, it ended ignominiously in 1813 after an attempt at forming an honour guard for Kaiser Franz and Grand Duke Constantine of Russia resulted in a botched set of maneuvers and an utter failure to from up correctly at the head of the column much to Constantine's bemusement and to the dishonour of the Reiters.

I've also come across mentions a company sized Corps of "Chasseurs d'Albini" or "Albinische Jager-Korps" named for the Imperial general Joseph, freiherr von Albini. It was incorporated into the Frankfurt troops in 1806 but it was initially raised as part of the army of the Elector of Mainz. Regardless it was seen as being highly ill-disciplined and of little military value, it was also quite small - only 30 men in August 1805.

Local citizens were generally exempted from compulsory military service; sources of men were generally resident non-citizens, citizens of neighbouring states or the population of the rural sections of the Stadt. The officers were largely drawn from the patrician class and were generally well regarded, life was harder for the other ranks. Troop strength was kept at minimum levels to save costs resulting in low morale and troop quality - part of the 1727 regulation forbade local women to have relations with active duty soldiers, which couldn't have helped matters! This prohibition seems to have been largely ignored though. Troops were housed in barracks, instead being quartered around the city, off-duty time was poorly supervised but there was a lengthy list of prohibitions on soldiers activity with notably severe punishments for any infractions.

The Kreisecontingent companies were called up during the 7 Years War, as part of the Nassau-Weilburg Infantry regiment (named for its commander) with the Frankfurt troops being the largest single contingent. Uniforms were dark blue with white facings. The regiment fought at Rossbach in November 1757 and was regarded as an average regiment by the Prince de Soubise commanding the army. In September 1759 the regiment was trapped in Leipzig by the Prussians and was captured almost in full.

The Upper Rhine kreis was mobilised again in December 1789 to guard the Rhine from revolutionary France with the Frankfurters as part of the same regiment as previously (since renamed as the Solms Regiment) they saw action from 1792 but internal friction was causing issues within the Empire - the two large Hesse states had withdrawn their troops from the Upper Rhine Kreiscontingent and the remaining regiments were badly under-strength - these two regiments were destroyed during the defence of Mannheim in 1794. Following this there was increasing pressure from the burghers to reduce military spending - military spending had consumed 18% of the city's budget even in peacetime - and the armed forces were generally left to whither. The remaining forces, now uniformed in white with red facings, were merged with those of Prince-Primate Dalberg's other holdings.

Sources:

Frankfurter Sagen- und Geschichten-Buch - Carl Bertling

Geschichte der freien Stadt Frankfurt am Main von ihrem Anfang bis auf die neuesten Zeiten - Georg Lange

Beitrag zur Geschichte der älteren und neueren Verfassung der Reichs-Stadt Frankfurt - J. E. Gaudelius

Uniformenkunde - Richard Knotel

Napoleonic Uniforms (Volume III) - John Elting

European Armies of the French Revolution - Frederick Schneid